Semiconductor device and production method therefor

ABSTRACT

A semiconductor device has an MIS (metal-insulating film-semiconductor) structure, and a film mainly containing Al, O, and N atoms is used on a semiconductor. Alternatively, a semiconductor device has an MIS structure, and a film mainly containing Al, O, and N atoms is provided as a gate insulating film on a channel region between a source and a drain. Characteristics required of a gate insulating film of a 0.05 μm-gate-length-generation semiconductor transistor are satisfied. In particular, no fixed charge is included in the film, and impurity diffusion is reduced.

TECHNICAL FIELD

The present invention relates to an insulating film used for asemiconductor device and a production method for the insulating film.Furthermore, the present invention relates to a transistor elementincluding the insulating film, a semiconductor device, and a productionmethod therefor.

BACKGROUND ART

In recent years, in order to realize further higher packing densities,higher performances, and lower power consumptions of semiconductordevices, various researches have been carried out on improvements ofcharacteristics of insulating films used for semiconductor devices.Examples of insulating films used for semiconductor devices include gateinsulating films of transistors, element isolation films, capacitorinsulating films, interlayer insulating films, and passivation films,and researches have been carried out on insulating film materials inaccordance with respective places for uses.

With respect to the insulating film, attempts have been made to reducethe film thickness t_(ox) in terms of silicon oxide film, while theleakage current has been maintained. The film thickness t_(ox) in termsof silicon oxide film is defined as t_(ox)=ε_(SiO)t/ε with respect to athin film having a relative dielectric constant of ε and an actual filmthickness of t.

For example, a gate insulating film of a 0.05 μm-gate-length-generationsemiconductor transistor is required to have such an insulating propertythat the gate leakage current density is 1 A/cm² or less at a gatevoltage of 1.0 V when the film thickness is 1 nm or less in terms ofsilicon oxide.

Silicon oxide films have been previously used as gate insulating filmsof transistors. However, when a voltage of 1 V is applied to a siliconoxide film of 1 nm in thickness, even a direct tunneling current aloneexceeds 10 A/cm² and, therefore, the silicon oxide film cannot be used.

Consequently, an attempt has been made to apply a metal oxide having ahigh dielectric constant to the above-described insulating film. If nodeterioration occurs in the mobility of electron in a channel due to theapplication of the metal oxide to the gate insulating film, a reductionin voltage and low power consumption can be realized with no decrease inspeed of the transistor.

Besides the above-described insulating property, the followingcharacteristics are also required of the metal insulating materialapplied to the gate insulating film.

First, the interface to a silicon substrate (or a silicon substratecovered with any one of an extremely thin silicon oxide film, a siliconnitride film, and a silicon oxynitride film) is thermodynamically stablein order to prevent deterioration of the gate capacity due to a heattreatment in a transistor production process.

Second, no fixed charge is included in the film in order to suppress athreshold shift and a decrease in channel mobility of the transistor.

Third, no impurity diffusion in the film occurs in order to suppress athreshold shift and variations of the transistor.

From the viewpoint of the insulating property and the stability of aninterface to a silicon substrate, researches have been carried out onapplications of ZrO₂, HfO₂, silicates of them, lanthanoid oxides, andsilicates thereof to insulating films until now. ZrO₂, HfO₂, andlanthanoid oxides have high dielectric constants of at least 20 andexcellent insulating properties, but have the following problems in theuse as gate insulating films.

The crystallization temperatures are low, and are 400 to 600 degrees.Consequently, when a transistor is produced, noticeable crystallizationof an insulating film occurs due to a heat treatment in the formingprocess therefor. The flatness of the interface to silicon is therebydeteriorated, and the mobility in a channel is reduced. Furthermore,grain boundaries are randomly generated in the insulating film, and maycause variations in characteristics. If a material of an upper electrodereaches the silicon substrate through grain boundaries of crystals, themobility in a channel is reduced and variations occur in the thresholdshift during production of the transistor and, thereby, the transistorhas high possibility of deterioration in the performance.

When crystallization occurs randomly in a surface, etching cannot beuniformly carried out in etching of the gate insulating film. As aresult, some portions may not be etched and may remain in a source-drainregion. In addition, since zirconium, hafnium, and lanthanoid are metalmaterials and are not present previously in processes of siliconsemiconductor devices, many studies on contamination are necessary forintroduction of a silicon semiconductor production line. Consequently,ZrO₂, HfO₂, and lanthanoid oxides are not used readily as gateinsulating films of silicon semiconductor transistors as of now.

Silicates of ZrO₂, HfO₂, and lanthanoid oxides are most promising gateinsulating films because the band gap is large although the dielectricconstant is in the order of 10, and a phase separation-crystallizationtemperature is high and is at least 800 degrees. However, silicatescannot be formed into films by the use of vapor phase atomic layergrowth, while the vapor phase atomic layer growth is a most promisingfilm-forming process for gate insulating films from the viewpoint of theuniformity in film thickness and the like.

Many studies on contamination are also necessary for introduction of aproduction line because ZrO₂, HfO₂, and lanthanoid oxides are contained.Consequently, silicates of ZrO₂, HfO₂, and lanthanoid oxides are notreadily used as gate insulating films of silicon semiconductortransistors as of now.

Researches have also been carried out on application of aluminum oxide(Al₂O₃) to the gate insulating film since the crystallizationtemperature is high and Al has already been present in the siliconsemiconductor process. Al₂O₃ has a relatively high relative dielectricconstant (about 8 to 10 with respect to amorphous, and about 12 withrespect to single crystal) and a high insulating property, and theinterface to silicon is thermodynamically stable. Al₂O₃ has acrystallization temperature of at least 800 degrees. An element Al hasalready been commonly used for the silicon semiconductor process.Furthermore, Al₂O₃ can be formed into a film by the use of vapor phasegrowth atomic layer growth, while the atomic layer growth is a mostpromising film-forming process for a gate insulating film. For theabove-described reasons, Al₂O₃ has been actively researched as theabove-described gate insulating film in recent years.

A prototype of a fine transistor of 0.08 μm in gate length is reportedin International Electron Device Meeting Technical Digest 2000 P.223,wherein an aluminum oxide film is used as a gate insulating film, andpolycrystalline silicon is used as a gate electrode. However, thisincludes the following problems.

First, a negative charge is present in the aluminum oxide (Al₂O₃) film.A negatively electrified fixed charge is believed to be generated whenAl vacancy or interstitial oxygen is present in aluminum oxide. Althoughit is not clear which is the origin of this negative charge as of now,the mobility of electron in the channel is reduced due to this negativefixed charge when the aluminum oxide (Al₂O₃) film is applied to the gateinsulating film. In addition, the threshold of the transistor is shiftedas well. In this report, the mobility of electron actually becomesone-third of that in the case where a silicon oxide film is used, whilethis reduction is due to the negative fixed charge in the film.Therefore, no advantage is found with respect to the use of the aluminumoxide film as the gate insulating film.

Second, the aluminum oxide thin film has no resistance to diffusion ofboron. Consequently, the threshold of the transistor is shifted whenboron-doped polycrystalline silicon is used for the gate electrode as ina previous manner.

According to the description in Appl. Phys. Lett., Vol. 77 (2000),P.2207, when an annealing temperature was controlled at 800 degrees to850 degrees in a boron-doped polycrystalline silicon electrode/Al₂O₃ (8nm)/n-Si system, 8.8×10¹² B ions/cm² of boron diffused from thepolycrystalline silicon electrode to a silicon substrate through Al₂O₃,and the flat band voltage was significantly shifted and was 1.54 V.

Since a heat treatment is carried out at about 1,000° C. in theformation of a transistor, the threshold of the transistor shifts andvaries significantly due to diffusion of boron. Consequently, in thisreport, an extremely thin silicon oxynitride film of 0.5 nm or less inthickness was provided between the Al₂O₃ film and Si so as to block thediffusion of boron and, thereby, the flat band voltage difference wasable to be controlled at about 90 mV even after the annealing wascarried out at 800° C. to 850° C. However, as described above, themobility of electron in the channel is reduced due to the negative fixedcharge when Al₂O₃ is applied to the gate insulating film. Furthermore,when a silicon nitride film is used at the interface, the nitrogenconcentration becomes large at the interface to silicon and, thereby,the mobility of electron in the channel is reduced due to a positivefixed charge.

In Japanese Unexamined Patent Application Publication No. 7-193147, Alis introduced in a laminated film of SiO₂ and Si₃N₄, SIALON(Si₃N₄—AlN—Al₂O₃-based solid solution) is applied to a gate insulatingfilm and, thereby, the attempt is made to improve the insulatingproperty and the dielectric constant. However, the dielectric constantis reduced because large amounts of Si is contained.

As described above, in order that Al₂O₃ can be used as the gateinsulating film, the fixed charge must be reduced and the impuritydiffusion must be reduced in the film while the insulating property andthe stability of the interface to silicon are maintained. However,simultaneous realization of them is difficult as of now and, inparticular, no solution is known to reduce the fixed charge of Al₂O₃.

The present invention was made in consideration of these problems, andovercomes problems in the case where aluminum oxide is used as a gateinsulating material of a semiconductor transistor. It is an object ofthe present invention to provide a structure and a production method fora device including a metal insulating material thin film as a gateinsulating film, wherein the thin film satisfies characteristicsrequired of a gate insulating film of a 0.05 μm-gate-length-generationsemiconductor transistor, in particular, no fixed charge is included inthe film, and the impurity diffusion is reduced in the film.

DISCLOSURE OF INVENTION

A semiconductor device of the present invention has an MIS(metal-insulating film-semiconductor) structure, wherein thesemiconductor is a film mainly containing silicon or is silicon, and theinsulating film is a film mainly containing Al, O, and N atoms. Anothersemiconductor device of the present invention has an MIS(metal-insulating film-semiconductor) structure, wherein theabove-described semiconductor is a film mainly containing silicon or issilicon, and the above-described insulating film is (1-x)AlO_(3/2).xAlN(where 0<x<1).

Another semiconductor device of the present invention has a transistorincluding a source region, a drain region, a channel region, and a gateelectrode provided on the channel region with an insulating filmtherebetween, wherein the channel region is a film mainly containingsilicon or is silicon, and the insulating film is a film mainlycontaining Al, O, and N atoms. Another semiconductor device has atransistor including a source region, a drain region, a channel region,and a gate electrode provided on the channel region with an insulatingfilm therebetween, wherein the channel region is a film mainlycontaining silicon or is silicon, and the insulating film is(1-x)AlO_(3/2).xAlN (where 0<x<1).

Furthermore, the insulating film of the semiconductor device of thepresent invention has a nitrogen concentration ratio of at least 0.1percent and 10 percent or less in nonmetallic atoms, and the insulatingfilm has a film thickness of 5 nm or less.

A semiconductor device of the present invention has a transistorincluding a source region, a drain region, a channel region, and a gateelectrode provided on the channel region with a first insulating filmand a second insulating film therebetween, wherein the first insulatingfilm is a silicon oxide film or a silicon oxynitride film, and thesecond insulating film is a film mainly containing Al, O, and N atoms.Another semiconductor device has a transistor including a source region,a drain region, a channel region, and a gate electrode provided on thechannel region with a first insulating film and a second insulating filmtherebetween, wherein the first insulating film is a silicon oxide filmor a silicon oxynitride film, and the second insulating film is(1-x)AlO_(3/2).xAlN (where 0<x<1). Furthermore, in the semiconductordevice of the present invention, the first insulating film is present inthe side nearer to the channel region than is the second insulatingfilm, and the gate electrode is polycrystalline silicon, asilicon-germanium mixed crystal, or a metal nitride.

A production method for an insulating film of the present inventionincludes the step of depositing aluminum and the step of supplying anoxidizing agent and a nitriding agent simultaneously so as to oxidizeand nitride and, thereby, an aluminum oxynitride film is formed. Anothermethod includes the step of depositing aluminum and the step ofalternately supplying an oxidizing agent and a nitriding agent so as tooxidize and nitride and, thereby, an aluminum oxynitride film is formed.Another production method for an insulating film of the presentinvention includes the step of depositing aluminum oxide and the step ofnitriding the aluminum oxide and, thereby, an aluminum oxynitride filmis formed. Furthermore, in the production method for an insulating filmof the present invention, the nitrogen concentration ratio is at least0.1 percent and 10 percent or less in nonmetallic atoms.

A production method for a semiconductor device of the present inventionincludes the above-described step of forming the insulating film.Another production method for a semiconductor device of the presentinvention includes the step of forming a gate insulating film by theabove-described production method for the insulating film.

The inventors of the present invention found out that when a film mainlycontaining Al, O, and N atoms was used as an insulating film, anexcellent insulating property and an excellent stability of theinterface to silicon were exhibited, the dielectric constant was high,the fixed charge was low, and the impurity diffusion was able to besuppressed in the film. In the case where a film mainly containing Al,O, and N atoms was used, for example, in the case where aluminumoxynitride was prepared by addition of nitrogen to aluminum oxide andwas used, the negative fixed charge density in the film was reducedsignificantly. Furthermore, it was verified that the interface betweenaluminum oxynitride prepared by addition of nitrogen and siliconinterface (or a silicon substrate covered with any one of a siliconoxide film, silicon oxynitride film, and the like) was thermodynamicallystable, and the diffusion of impurity atoms was suppressed in the film.

With respect to the film mainly containing Al, O, and N atoms, thereason for the reduction of the negative fixed charge density in thefilm is not clear, but is estimated that nitrogen compensates defectsoriginating the fixed charge. Likewise, the reason for the reduction ofthe impurity diffusion in the film is not clear, but is estimated thatdefects originating the fixed charge are reduced by the addition ofnitrogen.

Here, the film mainly containing Al, O, and N atoms basically refers toa thin film of a solid solution of alumina ((Al₂O₃)) and aluminumnitride (AlN), and the chemical composition thereof can be representedby (1-x)AlO_(3/2).xAlN, where (1-x) and x represent a constituent ratioof Al₂O₃ and AlN, respectively, in the solid solution, and 0<x<1 holds.Such an insulating film is hereafter referred to as an aluminumoxynitride film.

In the embodiments, the composition ratio of an aluminum oxynitrideinsulating film may be represented by the ratio of nitrogen atoms tononmetallic elements (sum of oxygen and nitrogen) because directdetermination can be performed based on the composition analysis withSIMS and the like. The conversion is readily performed between this“nitrogen concentration ratio in nonmetallic elements” and the molarcomposition ratio x in the above-described expression of the solidsolution. The nitrogen concentration ratio in nonmetallic elements canbe converted based on a formula 2x/(3-x).

The structure of the above-described aluminum oxynitride film may beeither a crystal structure or an amorphous structure. However, inconsideration of the use as an insulating film in a semiconductordevice, the amorphous structure is preferable because no grain boundaryis present and the leakage current can be controlled at a low level.

The above-described expression of the solid solution shows an ideal casewhere alumina and aluminum nitride have respective stoichiometriccompositions and are brought into a solid solution. This elementconstituent ratio may deviate from an ideal state with respect to anactual film, especially, in an amorphous state. However, a deviation isallowable to some extent as long as the leakage current and the fixedcharge density are within allowable ranges. Specifically, when theamounts of oxygen and nitrogen relative to Al are fluctuated within therange of −10 percent to +5 percent of the amounts in the stoichiometriccompositions, almost no influence is exerted on the characteristics ofthe insulating film. The present invention includes the case wherealuminum oxynitride has a composition deviated to some extent asdescribed above.

Furthermore, atoms other than Al, O, and N may included as additives inthe aluminum oxynitride film of the present invention. However,additives other than Al, O, and N may have an influence on thecrystallization temperature, the dielectric constant, and the insulatingproperty of the film. Preferably, the crystallization temperature is notreduced, the dielectric constant is not reduced, and characteristics,e.g., the insulating property, are not deteriorated. Specifically, Zr,Hf, or a lanthanoid metal can be added to the aluminum oxynitride filmused in the present invention. Preferably, these metals are added in theforms of insulating oxides, e.g., ZrO₂ and HfO₂, and form solidsolutions with aluminum oxynitride serving as a matrix. At this time,deterioration of characteristics can be almost neglected when the amountof a metal oxide to be added is controlled at 20 percent or less of theentirety, preferably at 10 percent or less.

That is, when a metal oxide to be added is represented by MO, and amolar ratio of an additive is represented by y, desirably, y≦0.2 issatisfied in the compositional formula(1-y){(1-x)AlO_(3/2) .xAlN}.yMOand preferably, y≦0.1 is satisfied.

An insulating material, e.g., an insulating metal nitride, can be addedbesides the metal oxide.

The aluminum oxynitride film may be used as any insulating filmsapplicable to semiconductor devices. Here, the use as a gate insulatingfilm is mainly exemplified. However, the aluminum oxynitride film has ahigh dielectric constant and, therefore, may be used as a capacitanceinsulating film of DRAM, for example. A film mainly containing siliconcan be used as a semiconductor having the MIS structure.

The film mainly containing silicon may be a film including, germanium,carbon, and the like besides silicon. For example, when germanium andcarbon are added, desirably, germanium constitutes about 20 percent ofthe entirety or 10 percent or less, carbon constitutes about 1 percentor less of the entirety. For example, when the compositional formula isrepresented by Si(1-x-y).Gex.Cy, desirably, x and y satisfy 0≦x≦0.2 and0≦y≦0.01, respectively. In this manner, not only a silicon singlecrystal, a germanium or other group IV semiconductor, and a siliconsubstrate, but also even SOI can be used as the semiconductor.

The thickness of the substrate and the film thickness of thesemiconductor may be any thickness as long as the thicknesses areeffective in forming a transistor. By using the insulating film of thepresent invention, the fixed charge density can be reduced compared withthat in alumina. This is also stable when used as a gate insulating filmof a transistor because deterioration does not occur.

Desirably, the above-described insulating film has a nitrogenconcentration ratio of at least 0.1 percent and 10 percent or less.Here, the nitrogen concentration ratio indicates the proportion ofnitrogen atoms when nonmetallic atoms (the total amount mainly ofnitrogen atoms and oxygen atoms) in an aluminum oxynitride film isassumed to be 1. When the nitrogen concentration ratio is 0.1 to 10percent, the flat band shift is reduced, the diffusion of impurities issuppressed, and the fixed charge density is reduced.

The dielectric constant and the band gap of aluminum nitride (relativedielectric constant: 6.2, band gap: 6.2 eV) are smaller than those ofaluminum oxide (relative dielectric constant (amorphous): about 8 to 10,band gap: 8.3 eV). However, when the nitrogen concentration ratio is 0.1to 10 percent, the insulating property is hardly deteriorated, and theimpurity diffusion is suppressed.

When the nitrogen concentration ratio becomes 0.1 percent, the fixedcharge density is sharply decreased compared with that in alumina, andas the nitrogen concentration ratio approaches 3.5 percent, the tendencyto decrease becomes saturated and the fixed charge density approaches asubstantially minimum value. Therefore, desirably, the nitrogenconcentration ratio is controlled at 0.1 percent or more, and preferablyat 3.5 percent or more. If the nitrogen concentration ratio exceeds 10percent, the leakage current density is sharply increased, thedielectric constant is sharply decreased, and the crystallizationtemperature is sharply decreased. Desirably, the nitrogen concentrationratio is controlled at 10 percent or less from the above-describedreasons.

Furthermore, when an upper electrode is impurity-containingpolycrystalline silicon, preferably, the nitrogen concentration ratio is5 percent or less since the difference is further decreased between theflat band voltage before a heat treatment and the flat band voltageafter the heat treatment. When this is converted to x in the expression(1-x)AlO_(3/2).xAlN, the nitrogen concentration ratio of 0.1 to 10percent corresponds x of about 1.50×10⁻³ to 1.43×10⁻¹ (0.15 percent to14.3 percent).

The crystallization temperature is decreased with increase in thenitrogen concentration. However, when the nitrogen concentration ratiois 5 percent or less, the crystallization temperature is maintained at800 degrees or more. Furthermore, even in the case where crystallizationoccurs, grain boundaries do not develop adequately, and the surfaceflatness is not deteriorated.

Desirably, the insulating film has a film thickness of 5 nm or less.This is because in the case where nitrogen plasma is used and thenitrogen plasma can compensate defects effectively, active species inthe nitrogen plasma are rapidly deactivated in the neighborhood of thedepth exceeding 5 nm and, therefore, nitrogen does not enter the regionlocated at a depth exceeding 5 nm.

The insulating film may be a laminated film of at least two types, andin this case, an aluminum oxynitride film is essentially included as anyone of the layers. A silicon oxide film or a silicon oxynitride film maybe included as a layer other than the aluminum oxynitride film.Preferably, the insulating film is a laminated film of two types, andincludes a first insulating film and a second insulating film.Preferably, the second insulating film is an aluminum oxynitride film,and the first insulating film is a silicon oxide film or a siliconoxynitride film.

When the insulating film is used as a gate insulating film, desirably,the first insulating film, the second insulating film, and the gateelectrode are disposed in that order from the channel region side. Inthis case, desirably, the thickness of the first insulating film isextremely small and is in the order of 0.5 nm. This is because anextremely small thickness prevents the reduction of an effectiverelative dielectric constant of the aluminum oxynitride film, theinterface state density at the interface to silicon is reduced and,thereby, the reduction of mobility of electrons in the channel issuppressed, so that further speedup of the transistor can be realized.

Here, a fresh Si-containing layer may be formed at the interface betweenthe first insulating film and the second insulating film through a heattreatment at a high temperature and the like. However, the content of Siis such a small amount that a reduction of the dielectric constant dueto Si can be neglected and, therefore, even when such a layer is formed,a high dielectric constant can be maintained because a layer simplycomposed of Al, O, and N is present in at least a part of the insulatingfilm of the present invention.

In the case where aluminum oxynitride is used as a gate insulating filmof a silicon semiconductor transistor, when polycrystalline silicon or asilicon-germanium mixed crystal is used as the gate electrode, a siliconsemiconductor transistor can be realized, wherein aluminum oxynitridehas high resistance to impurity diffusion and, thereby, almost nothreshold shift nor reduction of mobility occurs due to diffusion ofdoping elements, e.g., boron.

When aluminum oxynitride is used as a gate insulating film of a siliconsemiconductor transistor, and a metal nitride is used as a gateelectrode, the work function of the metal nitride film does not varyduring the production process for the transistor after the gateelectrode is formed. This is because defects are compensated in aluminumoxynitride and, therefore, no path is present for nitrogen to come outof the metal nitride, so that the amount of nitrogen is kept constant ina portion of the metal nitride while the portion is in contact withaluminum oxynitride. Consequently, the flexibility is increased in theproduction process for the transistor after the gate electrode isformed, and the productivity is increased.

Furthermore, when this insulating film is used as a gate insulating filmof a transistor, any known material can be used as a side wall used forthe side wall of the gate electrode, and a silicon oxide film, siliconoxynitride film, and the like can be used.

Aluminum oxynitride is formed by reactive sputtering and, thereby,amorphous aluminum oxynitride can be efficiently deposited on a siliconsurface. Any previously known methods may be used as the reactivesputtering method. Any apparatus, e.g., various plasma generationapparatuses of parallel-plate type, narrow-gap type, magnetron type, andtriode type, can be used as the reactive sputtering apparatus.

Aluminum, aluminum oxide, aluminum nitride, aluminum oxynitride, and thelike can be used as a target of the reactive sputtering in the formationof the insulating film of the present invention. However, any materialcan be used as long as the material can efficiently introduce Al,oxygen, and nitrogen. The nitrogen concentration ratio in aluminumoxynitride can readily be controlled by varying the mixing ratio of amixed gas of oxygen and nitrogen supplied into a chamber.

When aluminum is used as the target, at least nitrogen and oxygen areintroduced into the chamber, and if necessary, other various supplygases, such as rare gases, e.g., Ar, may be introduced in order togenerate and maintain plasma or to increase a sputtering efficiency.

When aluminum oxide is used as the target, at least a nitrogen gas isintroduced, and if necessary, a rare gas and oxygen can be supplied.When aluminum nitride is used as the target, at least oxygen isintroduced as a supply gas, and if necessary, a rare gas and nitrogencan be supplied. When aluminum oxynitride is used as the target, atleast a rare gas is introduced as a supply gas, and if necessary, amixed gas of nitrogen and oxygen can be supplied. In this case, anamorphous aluminum oxynitride film can be formed at a substratetemperature (400° C. or less) commonly used in reactive sputtering.

Since aluminum oxynitride formed here is added as a constituent elementof aluminum oxynitride, an elimination step after the formation isunnecessary in contrast to the case where aluminum oxynitride isprepared by nitriding aluminum oxide.

Oxygen vacancies included in the film can be effectively compensated byheat-treating the aluminum oxynitride film formed. Here, the heattreatment refers to annealing of aluminum oxynitride formed on a siliconsurface in an atmosphere containing oxygen.

Alternatively, an aluminum oxynitride film can be formed on a siliconsurface by an atomic layer deposition method (ALD method), and theresulting aluminum oxynitride film has a nitrogen concentration ratio ofat least 0.1 percent and 10 percent or less in nonmetallic atoms. In theALD method, the step of supplying an aluminum raw material on a siliconsurface, followed by adsorbing aluminum, and the step of supplying anoxidizing agent and a nitriding agent while the mixing ratio isadjusted, followed by carrying out oxidation and nitriding, are repeatedsimultaneously or alternately.

According to this ALD method, adsorption of a raw material precursor ona wafer surface and an oxidation reaction are repeated alternately and,thereby, excellent homogeneity in the wafer can be maintained, while thehomogeneity is required of a gate insulating film. Any material is usedas a raw metal material in the adsorption of aluminum as long asaluminum can be adsorbed effectively, and an Al-containing organicmetal, e.g., trimethylaluminum, may be included.

Likewise, oxidizing agents and nitriding agents may include any materialas long as the material can adequately carry out oxidation andnitriding. For example, H₂O or oxygen can be used as the oxidizingagent, and ammonia can be used as the nitriding agent. With respect tothe oxynitriding reaction, for example, a mixture of H₂O serving as theoxidizing agent and ammonia serving as the nitriding agent may be used,oxygen plasma may be used as the oxidizing agent, and hydrazine ornitrogen plasma may be used as the nitriding agent.

After trimethylaluminum is applied, water or oxygen plasma may beapplied so as to carry out one layer atomic deposition of aluminumoxide. Subsequently, nitrogen plasma may be applied so that nitrogen isincluded in the film. In a manner similar to that in the reactivesputtering, the resulting aluminum oxynitride may be heated.

Alternatively, an aluminum oxynitride film may be formed by the step offorming aluminum oxide on a silicon surface and, thereafter, nitridingthe aluminum oxide. Any method, e.g., reactive sputtering or an ALDmethod, may be used for forming aluminum oxide as long as the method canform aluminum oxide. With respect to nitriding methods, variousnitriding means are applicable, wherein nitrogen plasma, that is, amixture of nitrogen ions and nitrogen radicals, ammonia, hydrazine, orthe like is used.

In particular, when nitrogen plasma is used as the nitriding method,nitrogen active species do not reach a silicon substrate due to highreactivity of the plasma. Consequently, the formation of the fixedcharge due to nitriding of the silicon substrate can be prevented, andthe fixed charge in the aluminum oxide film can be compensated.According to this method, a transistor can be formed, wherein nothreshold shift occurs, the voltage is low, and the power consumption islow.

When the nitriding is carried out by the use of nitrogen plasma, thenitrogen concentration ratio in the film can be controlled by varyingthe pressure. The nitrogen concentration ratio increases as the plasmageneration pressure is reduced. This is because as the plasma generationpressure is reduced, the activation efficiencies of nitrogen radicalsand the like in the plasma are increased because of an increase in meanfree path and an increase in plasma electron temperature. Through theuse of this, for example, the pressure is varied from 10⁻Pa to 1 Pa and,thereby, the nitrogen concentration ratio in the aluminum oxynitridefilm can readily be controlled at within the range of 1 to 10 percent,so that the fixed charge can be reduced and the impurity diffusion canbe suppressed effectively.

It was made clear that the nitrogen concentration ratio in the aluminumoxynitride film was able to be controlled at within a preferable rangeby a heat treatment after the nitriding step as well. The reason forthis is believed that nitrogen is readily eliminated from the aluminumoxynitride film by the heat treatment. Desirably, the temperature of theheat treatment is 600° C. or less in order to prevent oxidation of Si,while the oxidation temperature of Si is 600° C. By carrying out thisheat treatment, an aluminum oxynitride film can be formed to have anitrogen concentration ratio of at least 0.1 percent and 10 percent orless in nonmetallic atoms. When nitrogen is removed by the heattreatment, as described above, no silicon nitride film is generated atthe interface between silicon and the gate insulating film due to theheat treatment in the transistor formation process after deposition ofthe gate insulating film. Therefore, the formation of the fixed chargedue to nitriding of the silicon substrate can be prevented, so that ahigh-speed transistor can be formed wherein no threshold shift occurs.

Japanese Unexamined Patent Application Publication No. 64-23571discloses a structure in which an oxide layer covers a surface or a sidesurface of an aluminum nitride film formed on a group III to V compoundsemiconductor. The oxide layer is provided as a protective layer toprevent penetration of water from the upper side or the side surface ofthe gate insulating film because aluminum nitride formed on the groupIII to V compound semiconductor has hygroscopicity. Therefore, thisstructure does not realize the insulating property of the gateinsulating film, the reduction of fixed charge, nor the stability ofinterface to silicon mainly targeted by the present invention, and isessentially different from the present invention, while the insulatingfilm is formed on silicon in the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the composition dependence of the dielectricconstant of a metal oxynitride thin film formed on a silicon oxide film,used in an embodiment of the present invention.

FIGS. 2A and 2B are graphs each showing the composition dependence ofthe fixed charge in a metal oxynitride thin film formed on a siliconoxide film, used in an embodiment of the present invention.

FIG. 3 is a graph showing the composition dependence of the leakagecurrent of a metal oxynitride thin film formed on a silicon oxide film,used in an embodiment of the present invention.

FIG. 4 is a graph showing the composition dependence of the change inflat band voltage, used in an embodiment of the present invention,wherein boron passes through a metal oxynitride thin film during a heattreatment and, thereby, the change is effected.

FIG. 5 is a graph showing the composition dependence of thecrystallization temperature of a metal oxynitride thin film formed on asilicon oxide film, used in an embodiment of the present invention.

FIG. 6 is a system diagram of an atomic layer deposition apparatus usedin the present invention.

FIGS. 7A to 7D are sectional views showing the structure and theproduction steps of a semiconductor device used in an embodiment of thepresent invention.

FIG. 8 is a graph showing the gate leakage characteristic of asemiconductor device used in an embodiment of the present invention.

FIGS. 9A to 9E are sectional views showing the structure and theproduction steps of a semiconductor device used in an embodiment of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment)

In the first embodiment, an aluminum oxynitride film was deposited on asilicon substrate by reactive sputtering. A magnetron sputteringapparatus was used as a reactive sputtering apparatus. An n-type siliconsubstrate (100) was used as a substrate. Aluminum was used as a target,the distance between the target and the substrate was controlled atabout 10 cm, and reactive sputtering was carried out, so as to form analuminum oxynitride thin film of 5 nm in thickness. The pressure in achamber was 5×10⁻⁵ Pa when the sputtering was not carried out.

An oxygen-nitrogen mixed gas was supplied during the film formation.Preferably, the total pressure of the mixed gas is controlled at a suchlevel that the mean free path of the gas becomes in the order of afraction of the distance between the target and the wafer to reduceimplantation of sputtered particles into the substrate. The mean freepath L (cm) of the gas is represented by L=1.33/P where a pressure is P(Pa). Preferably, the gas pressure is controlled at about 0.5 Pa or morewhen the distance between the target and the wafer is 10 cm, forexample. After the gas pressure was stabilized, a high frequency of13.56 MHz was applied between the target and a susceptor (including aheater) carrying the substrate with an RF power supply. In the reactivesputtering, heating of the substrate was not carried out, anitrogen-oxygen mixed gas was supplied, the pressure in the chamber wascontrolled at 0.6 Pa, and the power of the high frequency of the RFpower supply (frequency 13.56 MHz) was 500 W. In order to compensateoxygen vacancies, annealing was carried out at a substrate temperatureof 400° C. at 1 atmosphere for 10 minutes in an oxygen atmosphere. Afterthe annealing was carried out, a silicon polycrystal film was formed asan upper electrode on the aluminum oxynitride, boron was implanted intothe resulting film, and activation was carried out by a heat treatment(700° C.) in nitrogen.

The fixed charge density of the aluminum oxynitride thin film wasdetermined by calculation from the flat band voltage shift, while thealuminum oxynitride thin film was formed by the reactive sputtering. Thefixed charge density was 10⁻⁸ C/cm² or less, and it was made clear thatthe fixed charge was reduced compared with the fixed charge in the casewhere alumina was used. The leakage current density of this film wasdetermined and was 10⁻² A/cm² or less. This leakage current density wasdetermined during the application of a voltage larger than the flat bandvoltage by 1 V. The result indicated that the leakage current wasreduced in the aluminum oxynitride film. Furthermore, the dielectricconstant of this film was determined and was a value in the order of9.5. Consequently, a high-dielectric constant characteristic was clearlyrealized compared with the dielectric constant of aluminum oxide.

The crystallization temperature of this film was at least 800° C. and,therefore, a high temperature was achieved with respect to thecrystallization temperature. The relationships between the nitrogenconcentration ratio and the relative dielectric constant, the fixedcharge density, and the leakage current density will be described indetail in a second embodiment.

In the description up to this point, aluminum was used as the target.However, when any one of aluminum oxide, aluminum nitride, and aluminumoxynitride was used as the target, the result was similar to that in thecase where aluminum was used as the target.

(Second Embodiment)

The second embodiment is almost the same as the first embodiment exceptthat a first insulating film is provided between the silicon substrateand the aluminum oxynitride and a laminated film was thereby prepared.The second embodiment is different from the first embodiment in thepoint that an n-type silicon substrate (100) with 0.5 nm of siliconoxide film (the first insulating film) is used as the substrate.

Aluminum was used as a target, the distance between the target and thesubstrate was controlled at about 10 cm, and reactive sputtering wascarried out, so as to deposit an aluminum oxynitride thin film of 5 nmin thickness. The pressure in a chamber was 5×10⁻⁵ Pa when thesputtering was not carried out. An oxygen-nitrogen mixed gas wassupplied during the film formation, the pressure in the chamber wascontrolled at 0.6 Pa, and the power of the high frequency of the RFpower supply (frequency 13.56 MHz) was 500 W. In order to compensateoxygen vacancies, annealing was carried out at a substrate temperatureof 400° C. at 1 atmosphere for 10 minutes in an oxygen atmosphere. Afterthe annealing was carried out, a silicon polycrystal film was formed asan upper electrode on the aluminum oxynitride, boron was implanted intothe resulting film, and activation was carried out by a heat treatment(700° C.) in nitrogen.

With respect to the aluminum oxynitride thin film formed by the reactivesputtering, the composition dependence of each of the amount of fixedcharge, the impurity diffusion, the crystallization temperature, theleakage current, and the relative dielectric constant was examined.Here, the thickness of the aluminum oxynitride was controlled at asubstantially constant and was 5 nm. The films were formed to have aconstant film thickness in consideration of difference in film formationtime depending on the compositional ratio.

FIG. 1 shows the composition dependence of the relative dielectricconstant in the case where the activation annealing temperature is 700°C. With respect to the relative dielectric constant, the capacitance ofthe entire laminated film was measured, and the relative dielectricconstant of the aluminum oxynitride was determined from the measurementvalue, while the laminated film was formed on the silicon substrate andwas composed of a silicon oxide film serving as the first insulatingfilm and an aluminum oxynitride film serving as the second insulatingfilm.

As is clear from FIG. 1, the dielectric constant is decreased sharplywhen the nitrogen concentration ratio exceeds 10 percent on thecondition that the film thickness is constant. In FIG. 1, no data isshown in the region where the nitrogen concentration ratio exceeds 75percent because this region is not suitable for the object of thepresent invention on the grounds that, for example, the insulatingproperty is significantly inferior to that of aluminum oxide in thisnitrogen concentration region and the crystallization temperature issignificantly decreased.

FIG. 2A shows the composition dependence of the fixed charge density inthe aluminum oxynitride thin film, while the fixed charge density iscalculated from the flat band voltage shift. FIG. 2B is a magnifieddiagram of a part of FIG. 2A. As is clear from FIGS. 2A and 2B, thefixed charge density is decreased as a result of adding nitrogen toaluminum oxide. As is clear from FIG. 2B, the fixed charge is decreasedsharply compared with that in alumina when the nitrogen concentrationratio reaches 0.1 percent and, furthermore, the fixed charge density isdecreased to less than or equal to about one-tenth of that in aluminawhen the nitrogen concentration ratio reaches 1 percent. In addition,the fixed charge density approaches a minimum value when the nitrogenconcentration ratio reaches 3.5 percent, and takes on the minimum valuewhen the nitrogen concentration ratio is in the neighborhood of 5percent. Thereafter, even when the nitrogen concentration ratio is kepton increasing, the fixed charge density is not decreased. When a reducedfilm thickness is discussed based on the composition dependence of therelative dielectric constant shown in FIG. 1, the reduced film thicknessis increased as the nitrogen concentration ratio is increased.Consequently, when the reduced film thickness is required to becontrolled at a constant value, the actual film thickness is decreased,and the fixed charge density is predicted to be decreased accompanyingthat. However, the amount of change of the relative dielectric constantdue to the composition is orders of magnitude smaller than the amount ofchange of the fixed charge density shown in FIGS. 2A and 2B. Therefore,even when the reduced film thickness is controlled at a constant valueunder predetermined conditions, the fixed charge can be decreasedadequately as long as the nitrogen concentration ratio is 0.1 to 10percent.

FIG. 3 shows the composition dependence of the leakage current. Theleakage current density was determined during the application of avoltage larger than the flat band voltage by 1 V. It is clear that theleakage current density is increased sharply when the nitrogenconcentration ratio becomes at least 10 percent. With respect to this aswell, in consideration of the fact that the actual film thickness isdecreased as the nitrogen concentration ratio is increased when thereduced film thickness is required to be controlled at a constant value,the leakage current density is believed to be increased sharply with theslope larger than the slope of the solid line. However, in a mannersimilar to that described above, the amount of change of the relativedielectric constant is orders of magnitude smaller than the amount ofchange of the leakage current density shown in FIG. 3. Therefore, evenwhen the reduced film thickness is controlled at a constant value underpredetermined conditions, the leakage current can be decreased when thenitrogen concentration ratio is controlled at 0 to 10 percent.

In FIGS. 2A and 3, no data is shown in the regions where the nitrogenconcentrations exceed 75 percent. This regions are not suitable for theobject of the present invention on the grounds that, for example, theinsulating property is significantly inferior to that of aluminum oxidein this nitrogen concentration region and the crystallizationtemperature is significantly decreased.

FIG. 4 shows the composition dependence of the difference between theflat band voltage at an activation annealing temperature of 700° C. andthat at 800° C. As is clear from FIG. 4, substantially no flat banddifference is recognized as a result of adding 5 percent or less ofnitrogen to aluminum oxide. The flat band voltage difference is 0.6 V orless when the nitrogen concentration ratio is within the range of 0.1 to10 percent, and furthermore, the flat band voltage difference is 0.3 Vor less when the nitrogen concentration ratio is within the range of 1to 5 percent. Consequently, it is clear that the diffusion of impuritiesis suppressed. The effect of suppressing the diffusion of boron isreduced when the nitrogen concentration ratio exceeds 5 percent. Thereason is believed that grain boundaries bonding the upper electrode andthe silicon substrate are formed due to noticeable crystallization ofthe aluminum oxynitride thin film in the above-described nitrogenconcentration region, and boron diffuses into the silicon substratethrough the grain boundaries, as described below. With respect to thisas well, the shape of the graph may be changed to some extent when thereduced film thickness is controlled at predetermined conditions.However, the amount of change in the relative dielectric constant issmall and, therefore, no influence is exerted. In FIG. 4 as well, nodata is shown in the region where the nitrogen concentration ratioexceeds 75 percent. The reason is the same as that described above. Thisregion is not suitable for the object of the present invention on thegrounds that, for example, the insulating property is significantlyinferior to that of aluminum oxide in this nitrogen concentration regionand the crystallization temperature is significantly decreased.

The composition dependence of the crystallization temperature of thealuminum oxynitride prepared was determined by rapidly heating thealuminum oxynitride for 1 minute in nitrogen and carrying out an X-raydiffraction measurement. The results thereof are shown in FIG. 5. TheX-ray used was CuKα (wavelength: 0.15418 nm). The film thickness of thealuminum oxynitride film used for the measurement was 25 nm. As is clearfrom FIG. 5, the crystallization temperature becomes 800° C. or lesswhen the nitrogen concentration exceeds 10 percent in nonmetallic atomsand, therefore, phase separation of aluminum oxynitride occurs byheating to at least 800° C. while aluminum oxynitride is crystallized.Consequently, in the case where the nitrogen concentration exceeds 10percent in nonmetallic atoms, occurrence of the above-described impuritydiffusion is estimated.

In the case where the laminated layer included the first insulating filmand the second insulating film, even when a silicon oxynitride film orother insulating film was used as the first insulating film in place ofthe silicon oxide film, as in the present embodiment, thermodynamicstability of the interface to silicon was verified through the use of aTEM observation. Therefore, it was made clear that the interface statedensity was reduced at the interface to silicon when the firstinsulating film was provided at the interface between silicon andaluminum oxynitride so as to prepare a laminated film.

In the description up to this point, aluminum was used as the target.However, when any one of aluminum oxide, aluminum nitride, and aluminumoxynitride was used as the target, the result was similar to that in thecase where aluminum was used as the target.

(Third Embodiment)

In the third embodiment, aluminum oxynitride was formed by an atomiclayer deposition method (ALD method). FIG. 6 shows a conceptual diagramof an atomic layer deposition apparatus with a plasma source, used inthe present embodiment. The present apparatus is composed of a sampletreatment chamber 101 and an exchange chamber 102, and a plurality ofwafers 103 can be contained in the exchange chamber 102. A gate valve104 is provided between the sample treatment chamber 101 and theexchange chamber 102, and gases are exhausted from the chambers withexhaust systems 105 and 106, respectively, composed of a plurality ofpumps.

A heater 107 is disposed immediately below a wafer 108 carried from theexchange chamber to the sample treatment chamber, and heats the wafer toa predetermined temperature. An ECR plasma source 109 is disposed abovethe wafer 108 in the upper portion of the chamber, and serves as aplasma source.

In order to form a metal oxynitride on the upper surface of the wafer, ametal material gas, an oxidizing agent, and a nitriding agent isintroduced with gas supply systems 110 to 121.

The gas supply systems 110 to 121 are composed of metal material gassupply systems 110 to 113, oxidizing agent supply systems 114 to 117,and nitriding agent supply systems 118 to 121. Each of the gas supplysystems has basically the same configuration, and is composed of a rawmaterial cylinder 110, 114, or 118, stop valves 111 and 113, 115 and117, or 119 and 121, and a mass flow controller 112, 116, or 120. Theraw material cylinder 110 contains a metal material gas oftrimethylaluminum, the raw material cylinder 114 contains the oxidizingagent of water or oxygen, and the raw material cylinder 118 contains thenitriding agent of ammonia, hydrazine, or nitrogen. The raw materialcylinder 110 of trimethylaluminum is heated to 45° C. in order that themass flow controller 112 is normally operated.

A film formation procedure will be described. In the deposition ofaluminum oxide by the ALD method, usually, trimethylaluminum and waterserving as the oxidizing agent are applied alternately. In thedeposition of aluminum oxynitride by the ALD method, ammonia is added towater serving as the oxidizing agent, and an application to thesubstrate is carried out.

An n-type silicon substrate (100) was used as a substrate. Thedeposition apparatus was evacuated at a reduced pressure of 10⁻⁵ Pa orless. Trimethylaluminum was applied at a partial pressure of 1 P for 10seconds at a substrate temperature of 300° C. and, subsequently, a mixedgas of H₂O and ammonia was applied at 1 Pa for 10 seconds. Thedeposition per cycle was about 0.1 nm, and deposition was repeated 20times so as to deposit 2 nm of aluminum oxynitride. With respect to thepresent method, it was verified that the amount of nitrogen in the filmwas able to be changed by voluntarily changing the mixing ratio of H₂Oand ammonia. In the case where the film was formed by the atomic layerdeposition method through the above-described procedure, nitrogen wasadded as a constituent element of aluminum oxynitride.

Thereafter, annealing was carried out at a substrate temperature of 400°C. at 1 atmosphere for 10 minutes in an oxygen atmosphere. After theannealing was carried out, a silicon polycrystal film was formed as anupper electrode on the aluminum oxynitride, boron was implanted into theresulting film, activation was carried out by a heat treatment innitrogen, and the film characteristics were evaluated. The aluminumoxynitride deposited by the present method was verified to exhibit theproperties equivalent to those of the above-described aluminumoxynitride film deposited by the sputtering method when the nitrogencontents were the same.

(Fourth Embodiment)

In the fourth embodiment, an ALD method as in the third embodiment isused. However, plasma irradiation was carried out in the fourthembodiment in contrast to the third embodiment wherein the mixed gas ofH₂O and ammonia was used.

In carrying out the plasma irradiation as well, a silicon substrate asin the third embodiment was used, and the apparatus shown in FIG. 6 wasused. The plasma source was attached in the location immediately above awafer at a distance of 20 cm. After trimethylaluminum was applied at asubstrate temperature of 300° C., a mixed gas of O₂ and N₂ wasintroduced at a pressure of 10⁻¹ Pa, plasma was applied with 80 W ofpower for 15 seconds, and subsequently, each application was repeatedalternately, so as to deposit aluminum oxynitride. The deposition percycle was about 0.1 nm as in the above description, and deposition wasrepeated 20 times so as to deposit 2 nm of aluminum oxynitride. Withrespect to the present method, it was verified that the amount ofnitrogen in the film was able to be changed by voluntarily changing themixing ratio of O₂ and N₂. Thereafter, annealing was carried out at asubstrate temperature of 400° C. at 1 atmosphere for 10 minutes in anoxygen atmosphere. After the annealing was carried out, a siliconpolycrystal film was formed as an upper electrode on the aluminumoxynitride, boron was implanted into the resulting film, activation wascarried out by a heat treatment in nitrogen, and the filmcharacteristics were evaluated.

The aluminum oxynitride deposited by this method was verified to exhibitthe properties equivalent to those of the above-described aluminumoxynitride film deposited by the sputtering method when the nitrogencontents were the same. Alternatively, after trimethylaluminum wasapplied, water or oxygen was applied to deposit usual aluminum oxide.Subsequently, nitrogen plasma was applied in order that nitrogen wascontained in the film, and each application was repeated. As a result,it was verified that a comparable film was able to be deposited.

As described above, with respect to the aluminum oxynitride film formedby the ALD method as well, the fixed charge was able to be reduced, theimpurity diffusion was able to be suppressed, and a high-dielectricconstant characteristic was able to be realized.

(Fifth Embodiment)

The fifth embodiment is different from the third embodiment in the pointthat a silicon oxide film serving as the first insulating film isprovided between silicon and aluminum oxynitride so as to prepare alaminated film.

An aluminum oxynitride film was deposited by the ALD method. An n-typesilicon substrate (100) with 0.5 nm of silicon oxide film (the firstinsulating film) was used as the substrate. Trimethylaluminum wasapplied under the condition as in the third embodiment and,subsequently, a mixed gas of water and ammonium was applied. These wererepeated alternately, so as to deposit 2 nm of aluminum oxynitride.After annealing was carried out as in the third embodiment, a film ofupper electrode was formed, and the film characteristics were evaluated.The aluminum oxynitride deposited by the present method was verified toexhibit the properties equivalent to those of the above-describedaluminum oxynitride film deposited by the sputtering method when thenitrogen contents were the same.

It was also verified that since the aluminum oxynitride film was formedas a laminated film in the fifth embodiment, the interface state densitywas reduced and the reduction in mobility of channel electrons weresuppressed compared with those in the third embodiment. Similar resultswere attained when an insulating film, e.g., a silicon oxynitride film,other than the silicon oxide film was used as the first insulating filmbetween the aluminum oxynitride film and silicon. In the case whereplasma irradiation was carried out by the use of mixed plasma generatedfrom a mixed gas of O₂ and N₂, as in the fourth embodiment, instead ofthe mixed gas of H₂O and ammonia, a comparable aluminum oxynitride filmwas able to be prepared.

(Sixth Embodiment)

With respect to the method carried out in the sixth embodiment, aluminumoxide was deposited to have a predetermined film thickness and,subsequently, nitriding was carried out from the film surface, so as toform an aluminum oxynitride thin film. The aluminum oxide was formed bythe ALD method, and a nitriding reaction was carried out with nitrogenplasma (a mixture of nitrogen ions and nitrogen radicals).

An n-type silicon substrate (100) with 0.5 nm of silicon oxide film wasused as the substrate, and 2 nm of Al₂O₃ was deposited by the ALDmethod. Subsequently, nitrogen plasma was applied to the Al₂O₃ filmsurface. The substrate was introduced into the above-described apparatusequipped with a small ECR plasma source for a vacuum reaction so as tocarry out nitriding. The nitriding was carried out with 80 W of powerfor 10 minutes at a substrate temperature of 300° C. at a pressure of10⁻¹ Pa, and it was verified that nitrogen was contained in the film.

Thereafter, annealing was carried out at a substrate temperature of 400°C. at 1 atmosphere for 10 minutes in an oxygen atmosphere. As a resultof the above-described annealing, most of nitrogen contained in the filmwas eliminated, and only about 1 atomic percent of trace nitrogen wascontained regardless of the nitrogen content before annealing. Theelimination of nitrogen by the annealing was also observed in the casewhere ammonia or hydrazine was used for nitriding reaction.Consequently, a maximum amount of nitrogen becomes about 1 atomicpercent, while this nitrogen is added to aluminum oxide byafter-nitriding and is not readily eliminated.

Subsequently, a silicon polycrystal film was formed as an upperelectrode on the aluminum oxynitride, boron was implanted into theresulting film, activation was carried out by a heat treatment innitrogen, and the film characteristics were evaluated. The filmsubjected to the above-described nitriding treatment was verified toexhibit the properties equivalent to those of the aluminum oxynitridefilm deposited by the sputtering method and having a nitrogen content ofabout 1 percent.

Furthermore, the surface density of nitrogen atoms bonded to the siliconsubstrate becomes 1×10⁻¹⁰/cm² until the fixed charge in the film wasreduced to one-tenth the original value, and it was verified that thesurface density was able to be reduced. This is because nitridingactivation species do not reach the silicon substrate due to highreactivity of plasma. Consequently, according to this method, atransistor can be formed, wherein no threshold shift occurs, the voltageis low, and the power consumption is low.

The nitrogen plasma was used in the above-described embodiment. However,it was verified that similar effects were able to be exerted even whenthe nitriding was carried out by the use of ammonia or hydrazine in thenitriding reaction.

(Seventh Embodiment)

In the seventh embodiment, the change in nitrogen concentration ratiowas examined while the pressure was varied with respect to the methodfor nitriding aluminum oxide. This method was carried out in the sixthembodiment.

A nitrogen profile was examined in the case where 100 nm of aluminumoxide was deposited on a silicon substrate covered with a silicon oxidefilm and was nitrided with nitrogen plasma while the nitrogen pressurewas varied. The condition of the nitriding was similar to that in thethird embodiment. As the pressure was decreased, nitrogen was added todeeper portion because of a temperature increase of particles in plasma.However, nitrogen entered by only about 5 nm at a minimum pressure (10⁻¹Pa) required to stably generate the plasma. Consequently, with respectto addition of nitrogen to aluminum oxide, it was made clear that thefilm thickness of aluminum oxide had to be 5 nm or less in the casewhere nitrogen plasma was used because of high capability to compensatethe fixed charge.

In the above-described embodiment, the silicon substrate covered withthe silicon oxide film was used as the substrate. Similar results applyto the case where a silicon substrate or a silicon substrate coveredwith any one of extremely thin silicon oxynitride films is used.

(Eighth Embodiment)

FIG. 7D is a sectional view of an n-type transistor according to thefirst embodiment.

Element isolation regions 202 having an STI structure are provided on ann-type single crystal silicon substrate 201 having an impurityconcentration in the order of 5×10¹⁵ cm⁻³. A p well (not shown in thedrawing) is provided in an n-type transistor formation region. In thetransistor region isolated by the element isolation regions 202, ap-type channel impurity layer is provided (not shown in the drawing) inorder to control the threshold while having an impurity concentration inthe order of 5×10¹⁶ cm⁻³ and a source-drain region 203 is provided andis composed of an n-type diffusion layer having an impurityconcentration in the order of 5×10¹⁹ cm⁻³. A silicon oxynitride film 205(film thickness of 0.5 nm in terms of silicon oxide film) is provided ona channel region 204, and aluminum oxynitride (N/(O+N)=5 percent) 206 of1.2 nm in film thickness is further provided thereon.

A gate electrode 207 composed of polycrystalline silicon and WSi isprovided on the aluminum oxynitride film 206 while the gate electrode207 is self-aligning relative to the source-drain region 203. Eachsource-drain electrode 209 is provided and is electrically connected toeach source-drain region 203 through a contact hole provided in aninterlayer insulation film 208. Furthermore, the entirety is coveredwith a passivation film 210.

A production method for a single n-type transistor according to thefirst embodiment will be described sequentially with reference to FIGS.7A to 7D.

The surface of an n-type single crystal silicon substrate 201 is cleanedby a cleaning method through the use of a mixed aqueous solution ofhydrogen peroxide, ammonia, and hydrochloric acid. Since the object isto clean the surface of the single crystal silicon substrate 201,cleaning methods other than the above-described method may be used.

A p well is formed on the silicon substrate 201. Grooves are dug on thesubstrate 201 by an RIE (Reactive Ion Etch) method, and insulating filmsare embedded in the grooves, so as to form trench-type element isolationregions 202.

Subsequently, a silicon oxide film 211 of about 5 nm in thickness isformed, and channel ion implantation is carried out so as to form ap-type channel impurity layer (not shown in the drawing). Furthermore,activation of the p-type channel impurity layer is carried out by RTA(Rapid Thermal Anneal) at 800° C. for about 10 seconds (FIG. 7A).

The silicon oxide film is peeled off with hydrofluoric acid, and asilicon oxynitride film 205 (film thickness of 0.5 nm in terms ofsilicon oxide film) is formed. Thereafter, aluminum oxynitride(N/(O+N)=5 percent) is formed by a reactive sputtering method throughthe use of an aluminum target without heating the substrate, while thealuminum oxynitride serves as a metal oxynitride insulating film 206 of1.2 nm in film thickness. During the reactive sputtering, anitrogen-oxygen mixed gas is supplied, the pressure in a chamber iscontrolled at 0.6 Pa, and the power of the high frequency of an RF powersource (frequency 13.56 MHz) is 500 W. Not only aluminum, but also anyone of aluminum oxide, aluminum nitride, and aluminum oxynitride can beused as the target, and an ALD method or nitriding of aluminum oxide maybe used in place of the reactive sputtering method.

When the ALD method is used, the ALD method can be carried out as in theformation of aluminum oxynitride carried out in the above-describedthird, fourth, and fifth embodiments. When the nitriding of aluminum isused, the nitriding of aluminum can be carried out as in the sixthembodiment. Subsequently, annealing is carried out at 1 atmosphere at400° C. for 10 minutes in an oxygen atmosphere.

Polycrystalline silicon 207 is formed on the metal oxynitride insulatingfilm 206 by a low-pressure chemical vapor deposition method (LPCVD). Aphotoresist pattern (not shown in the drawing) is formed on thepolycrystalline silicon 207 (FIG. 7B). The polycrystalline silicon 207and the metal oxynitride insulating film 206 are patterned byanisotropic etching through the use of this photoresist pattern as anmask for etching.

The photoresist pattern, the polycrystalline silicon 207, and the metaloxynitride insulating film 206 are used as masks for ion implantation,impurity ions (arsenic) are implanted into the substrate 201 and,thereby, a source-drain region 203 is formed while the source-drainregion 203 is self-aligning relative to the polycrystalline silicon 207and the metal oxynitride insulating film 206 (FIG. 7C).

The photoresist pattern is removed, and a heat treatment (1 atmosphere,1000° C., 1 second, nitrogen atmosphere) is carried out to activate thesource-drain and the polycrystalline silicon 207. An interlayerinsulation film 208 is formed. Contact holes are formed to reach thesource-drain region 203 and the polycrystalline silicon 207, Co and TiN(not shown in the drawing) are deposited, and an RTA (Rapid ThermalAnneal) treatment is carried out in nitrogen at 700° C. for 10 seconds.Thereafter, this is patterned so as to form a source electrode, a drainelectrode 209, and a gate electrode composed of the polycrystallinesilicon 207 and WSi (FIG. 7D).

Annealing is carried out at 400° C. for 10 minutes in an atmosphere inwhich a ratio of nitrogen to hydrogen is 9:1. Finally, a passivationfilm 210 is formed all over the surface, so as to prepare a transistorshown in FIG. 7D.

With respect to the transistor in the eighth embodiment, the aluminumoxynitride has no fixed charge and has high resistance to impuritydiffusion and, therefore, no threshold shift nor deterioration of themobility in the channel were observed. The gate capacitance per unitarea was 3.7 [μF/cm²], and was larger than the gate capacitance per unitarea of 3.6 [μFrad/cm²] expected when the film thickness of the gateinsulating film was 1.0 nm in terms of silicon. That is, the filmthickness of the gate insulating film prepared was 1.0 nm or less interms of silicon.

FIG. 8 shows the gate voltage dependence of the gate leakage currentdensity of the transistor prepared in the above-described steps. Thegate leakage current density is 1 A/cm2 at a gate voltage of 1.0 V. Theinterface state density was 5×10¹⁰/cm² eV at the interface between thegate insulating film of the transistor prepared in the above-describedsteps and silicon. This value was substantially the same as theinterface state density at the interface between a silicon oxide filmformed by usual thermal oxidation and silicon. The operation of the thusprepared transistor was checked, and the transistor exhibited properoperation.

Even when a silicon-germanium mixed crystal was used as the gateelectrode in the above-described structure, the resulting effects weresimilar to those in the case where the polycrystalline silicon was used.

(Ninth Embodiment)

FIG. 9E is a sectional view of a single n-type transistor according tothe ninth embodiment. Element separation regions 302 having an STIstructure are provided on an n-type single crystal silicon substrate 301having an impurity concentration in the order of 5×10¹⁵ cm⁻³.

A p well (not shown in the drawing) is provided in an n-type transistorformation region. In the transistor region isolated by the elementisolation regions 302, a source-drain region 303 is provided and iscomposed of an n-type diffusion layer having an LDD (Lightly DopedDrain) structure with an impurity concentration in the order of 5×10¹⁹cm⁻³ (303 a) and an impurity concentration in the order of 5×10²⁰ cm⁻³(303 b). A p-type channel impurity layer (not shown in the drawing) isprovided in only the channel region 304 in order to control thethreshold while having an impurity concentration in the order of 5×10¹⁶cm⁻³.

A silicon oxide film 305 of 0.5 nm in film thickness is provided on thechannel region 304, and aluminum oxynitride (N/(O+N)=1 percent) 306 of1.2 nm in film thickness is further provided thereon. A gate electrode307 composed of TiN and W is provided on the aluminum oxynitride film306 while the gate electrode 307 is self-aligning relative to thesource-drain region 303 b.

A silicon oxide film 309 is provided between the metal oxynitrideinsulating film 306 and an interlayer insulating film 308. Eachsource-drain electrode 311 is provided and is electrically connected toeach source-drain region 303 through a contact hole provided in theinterlayer insulation films 308 and 310. Furthermore, the entirety iscovered with a passivation film 312.

A production method for a single transistor according to the ninthembodiment will be described sequentially with reference to FIGS. 9A to9D.

The surface of an n-type single crystal silicon substrate 301 is cleanedas in the first embodiment so as to form a p well.

Grooves are dug on the substrate 301 by the RIE method, and insulatingfilms are embedded in the grooves to form trench-type element isolationregions 302. Subsequently, a silicon oxide film 313 of about 5 nm inthickness is formed. A polycrystalline silicon film of about 300 nm infilm thickness is deposited all over this silicon oxide film in order toform a dummy gate pattern 314, and is processed into the dummy gatepattern by lithography and the RIE method. The polycrystalline siliconis used for the dummy gate pattern 314 because the etching selectivityis readily adjusted relative to the silicon oxide film 313 during theRIE and, thereby, etching damage to the silicon substrate 301 due to theRIE is readily reduced.

In order to form the LDD structure, about 4×10¹³ cm⁻² of phosphorous ionimplantation is carried out at 70 KeV, so that an n-type diffusion layer303 a is formed (FIG. 9A). After a silicon oxide film is deposited allover the surface, the RIE is carried out all over the surface, so that asilicon oxide film 309 of about 20 nm in thickness is formed on the sidesurface of a dummy gate pattern 305.

Subsequently, about 5×10¹⁵ cm⁻² of arsenic ion implantation is carriedout at 30 KeV to form an n⁺-type diffusion layer 303 b and to form theLDD structure (FIG. 9B). About 300 nm of silicon oxide film 308 isdeposited all over the surface by CVD, and annealing is carried out in anitrogen atmosphere at 750° C. for 30 minutes.

RTA is carried out in a nitrogen atmosphere at 950° C. for 10 seconds toactivate the ion implantation layer of the source-drain. Flattening iscarried out all over the surface by CMP (Chemical Mechanical Polishing)to expose the surface of the polycrystalline silicon film serving as thedummy gate pattern 314.

The exposed dummy gate pattern 314 is selectively removed by the RIE toexpose the surface of the silicon oxide film 313. Thereafter, only adesired channel region 304 is subjected to ion implantation through theuse of the interlayer insulation film 308 and the side wall insulatingfilm 309 as masks. With respect to the n channel transistor, in order tocontrol the threshold at about 0.7 V, about 5×10¹² cm⁻² of boron ionimplantation is carried out at 10 KeV, and a p-type channel region isselectively formed only in the channel region (FIG. 9C).

The silicon oxide film 313 is removed with diluted hydrofluoric acid,and a silicon oxide film 305 of 0.5 nm in film thickness is formed onthe exposed silicon substrate surface. Aluminum oxide of 1.2 nm in filmthickness is deposited all over the surface by the use of the ALD. Thealuminum oxide is subjected to after-nitriding with nitrogen plasma, soas to form aluminum oxynitride (N/(O+N)=1 percent) serving as the metaloxynitride insulating film 306.

In the ALD method used for depositing aluminum oxide, trimethylaluminumwas used as the raw material, and water or oxygen plasma was used as anoxidizing agent. Trimethylaluminum was applied at a substratetemperature of 300° C. at 1 Pa for 10 seconds, and succeedingly, anoxidizing agent was applied. These applications were repeatedalternately and, thereby, a film was formed. When plasma was used, theplasma was applied with 80 W of power for 15 seconds.

Here, the ALD method is used for depositing aluminum oxide. However,reactive sputtering may be used. In the case where nitrogen plasma isused for nitriding aluminum oxide, the nitriding is carried out at asubstrate temperature of 300° C. at 10⁻¹ Pa with 80 W of power for 10minutes. Ammonia or hydrazine may be used for nitriding. Here, aluminumoxynitride is deposited by nitriding aluminum oxide. However, aluminumoxynitride can be deposited directly as well.

Subsequently, activation of the channel region impurity is carried outby RTA at 800° C. for about 10 seconds in a nitrogen atmosphere. Thenumber of bonds is decreased through this step, wherein the bonds arenot terminated and are present at the interface between the siliconsubstrate 301 and the silicon oxide film 305. Consequently, reduction inthe interface state density can be realized. A heat treatment is carriedout at 400° C. in an oxygen atmosphere for 10 minutes to compensateoxygen vacancies in the aluminum oxynitride (N/(O+N)=1 percent) thinfilm.

Thereafter, TiN and W are formed all over the surface and serve as agate electrode 307. CMP is carried out all over the surface and,thereby, the gate electrode and the metal oxynitride insulating film 306are embedded in the groove, while the dummy gate has been removed fromthe groove, so that the gate electrode 307 is formed (FIG. 9D).

About 200 nm of silicon oxide film is deposited all over the surface andserves as an interlayer insulation film 310. Each contact hole is formedto reach the source-drain region 303.

Subsequently, Co and TiN (not shown in the drawing) and W are deposited,and are subjected to the RTA (Rapid Thermal Anneal) treatment innitrogen at 700° C. for 10 seconds. Thereafter, patterning is carriedout, so as to form a source electrode and a drain electrode 311.Annealing is further carried out at 400° C. for 10 minutes in anatmosphere in which a ratio of nitrogen to hydrogen is 9:1. Finally, apassivation film 312 is formed all over the surface, so as to prepare atransistor shown in FIG. 9E.

The inventors of the present invention verified that the performance ofthe thus formed transistor had characteristics equivalent to those ofthe transistor in the eighth embodiment, and the transistor exhibitedproper operation.

In the case where a metal nitride, TiN, was used for the gate electrodeas in the present invention, nitrogen was prevented from coming out ofthe metal nitride because aluminum oxynitride has high resistance toimpurity diffusion. Consequently, the threshold was prevented fromfluctuating due to the production process for the transistor followingthe formation of the gate electrode. Similar effects were observed inthe case where the gate electrode was a nitride of Ti, Zr, Hf, W, or Ta,a compound thereof, or a laminate of some of them.

When nitrogen was removed after aluminum oxide was nitrided, whilenitrogen was readily eliminated by a heat treatment, the siliconsubstrate was prevented from being nitrided during the process followingthe deposition of the gate insulating film. As a result, a transistorhaving an excellent mobility was surely realized.

INDUSTRIAL APPLICABILITY

According to the semiconductor device and the production method thereforof the present invention, it becomes possible to satisfy thecharacteristics required of a gate insulating film of a 0.05μm-gate-length-generation semiconductor transistor. In particular, thefixed charge in the film can be reduced, the impurity diffusion can besuppressed and, thereby, the threshold shift and deterioration of themobility can be prevented.

1. A semiconductor device having an MIS (metal-insulatingfilm-semiconductor) structure, wherein the semiconductor comprises afilm mainly containing silicon, and the insulating film comprises a filmmainly containing Al, O, and N atoms.
 2. A semiconductor device havingan MIS (metal-insulating film-semiconductor) structure, wherein thesemiconductor comprises a film mainly containing silicon, and theinsulating film comprises (1-x)AIO_(3/2).xAIN (where 0<x<1).
 3. Thesemiconductor device according to claim 1, wherein the semiconductorcomprises silicon.
 4. A semiconductor device comprising a transistorincluding a source region, a drain region, a channel region sandwichedby the source region and the drain region, and a gate electrode providedon the channel region with an insulating film therebetween, wherein thechannel region comprises a film mainly containing silicon, and theinsulating film comprises a film mainly containing Al, O, and N atoms.5. A semiconductor device comprising a transistor including a sourceregion, a drain region, a channel region sandwiched by the source regionand the drain region, and a gate electrode provided on the channelregion with an insulating film therebetween, wherein the channel regioncomprises a film mainly containing silicon, and the insulating filmcomprises (1-x)AIO_(3/)2.xAIN (where 0<x<1).
 6. The semiconductor deviceaccording to claim 4, wherein the channel region comprises silicon. 7.The semiconductor device according to claim 1, wherein the insulatingfilm has a nitrogen concentration ratio of at least 0.1 percent and 10percent or less in nonmetallic atoms.
 8. The semiconductor deviceaccording to claim 1, wherein the insulating film has a film thicknessof 5 nm or less.
 9. A semiconductor device comprising a transistorincluding a source region, a drain region, a channel region sandwichedby the source region and the drain region, and a gate electrode providedon the channel region with a first insulating film and a secondinsulating film therebetween, wherein the first insulating filmcomprises a silicon oxide film or a silicon oxynitride film, and whereinthe second insulating film comprises a film mainly containing Al, O, andN atoms.
 10. A semiconductor device comprising a transistor including asource region, a drain region, a channel region sandwiched by the sourceregion and the drain region, and a gate electrode provided on thechannel region with a first insulating film and a second insulating filmtherebetween, wherein the first insulating film comprises a siliconoxide film or a silicon oxynitride film, and wherein the secondinsulating film comprises (1-x)AlO_(3/2).xAIN (where 0<x<1).
 11. Thesemiconductor device according to claim 9, wherein the channel regioncomprises a film mainly containing silicon.
 12. The semiconductor deviceaccording to claim 9, wherein the channel region comprises silicon. 13.The semiconductor device according to claim 9, wherein the firstinsulating film is present in the side nearer to the channel region thanis the second insulating film.
 14. The semiconductor device according toclaim 4, wherein the gate electrode comprises polycrystalline silicon ora silicon-germanium mixed crystal.
 15. The semiconductor deviceaccording to claim 4, wherein the gate electrode comprises a metalnitride.
 16. A production method for an insulating film, comprising thesteps of: depositing aluminum; supplying an oxidizing agent and anitriding agent simultaneously so as to oxidize and nitride: and therebyforming an aluminum oxynitride film.
 17. A production method for aninsulating film, comprising the steps of: depositing aluminum;alternately supplying an oxidizing agent and a nitriding agent so as tooxidize and nitride; and thereby forming an aluminum oxynitride film.18. A production method for an insulating film, comprising the steps of:depositing aluminum oxide; nitriding the aluminum oxide; and therebyforming an aluminum oxynitride film.
 19. The production method for aninsulating film according to claim 16, wherein the insulating film has anitrogen concentration ratio of at least 0.1 percent and 10 percent orless in nonmetallic atoms.
 20. A production method for a semiconductordevice, comprising the step of forming an insulating film by the methodaccording to claim
 16. 21. A production method for a semiconductordevice, comprising the step of forming a gate insulating film by themethod according to claim
 16. 22. The semiconductor device according toclaim 2, wherein the semiconductor comprises silicon.
 23. Thesemiconductor device according to claim 5, wherein the channel regioncomprises silicon.
 24. The semiconductor device according to claim 2,wherein the insulating film has a nitrogen concentration ratio of atleast 0.1 percent and 10 percent or less in nonmetallic atoms.
 25. Thesemiconductor device according to claim 4, wherein the insulating filmhas a nitrogen concentration ratio of at least 0.1 percent and 10percent or less in nonmetallic atoms.
 26. The semiconductor deviceaccording to claim 5, wherein the insulating film has a nitrogenconcentration ratio of at least 0.1 percent and 10 percent or less innonmetallic atoms.
 27. The semiconductor device according to claim 2,wherein the insulating film has a film thickness of 5 nm or less. 28.The semiconductor device according to claim 4, wherein the insulatingfilm has a film thickness of 5 nm or less.
 29. The semiconductor deviceaccording to claim 5, wherein the insulating film has a film thicknessof 5 nm or less.
 30. The semiconductor device according to claim 10,wherein the channel region comprises a film mainly containing silicon.31. The semiconductor device according to claim 10, wherein the channelregion comprises silicon.
 32. The semiconductor device according toclaim 10, wherein the first insulating film is present in the sidenearer to the channel region than is the second insulating film.
 33. Thesemiconductor device according to claim 5, wherein the gate electrodecomprises polycrystalline silicon or a silicon-germanium mixed crystal.34. The semiconductor device according to claim 9, wherein the gateelectrode comprises polycrystalline silicon or a silicon-germanium mixedcrystal.
 35. The semiconductor device according to claim 10, wherein thegate electrode comprises polycrystalline silicon or a silicon-germaniummixed crystal.
 36. The semiconductor device according to claim 5,wherein the gate electrode comprises a metal nitride.
 37. Thesemiconductor device according to claim 9, wherein the gate electrodecomprises a metal nitride.
 38. The semiconductor device according toclaim 10, wherein the gate electrode comprises a metal nitride.
 39. Theproduction method for an insulating film according to claim 17, whereinthe insulating film has a nitrogen concentration ratio of at least 0.1percent and 10 percent or less in nonmetallic atoms.
 40. The productionmethod for an insulating film according to claim 18, wherein theinsulating film has a nitrogen concentration ratio of at least 0.1percent and 10 percent or less in nonmetallic atoms.
 41. A productionmethod for a semiconductor device, comprising the step of forming aninsulating film by the method according to claim
 17. 42. A productionmethod for a semiconductor device, comprising the step of forming aninsulating film by the method according to claim
 18. 43. A productionmethod for a semiconductor device, comprising the step of forming aninsulating film by the method according to claim
 19. 44. A productionmethod for a semiconductor device, comprising the step of forming a gateinsulating film by the method according to claim
 17. 45. A productionmethod for a semiconductor device, comprising the step of forming a gateinsulating film by the method according to claim
 18. 46. A productionmethod for a semiconductor device, comprising the step of forming a gateinsulating film by the method according to claim
 19. 47. A productionmethod for a semiconductor device, comprising the step of forming a gateinsulating film by the method according to claim
 20. 48. A semiconductordevice as claimed in claim 2, wherein the insulating film isSi(1-x-y).Gex.Cy.
 49. A semiconductor device as claimed in claim 2,wherein the insulating film is (1-y){(1-x)AlO_(3/2).xAIN}.yHfO₂ (0<x<1,y is equal to or smaller than 0.2).
 50. A semiconductor device asclaimed in claim 4, wherein the insulating film is(1-y){(1-x)AlO_(3/2).xAIN}.yHfO₂ (0<x<1, y is equal to or smaller than0.2).
 51. A semiconductor device as claimed in claim 5, wherein theinsulating film is (1-y){(1-x)AlO_(3/2).xAIN}.yHfO₂ (0<x<1, y is equalto or smaller than 0.2).
 52. A semiconductor device as claimed in claim9, wherein the insulating film is (1-y){(1-x)AlO_(3/2).xAIN}.yHfO₂(0<x<1, y is equal to or smaller than 0.2).
 53. A semiconductor deviceas claimed in claim 10, wherein the insulating film is(1-y){(1-x)AlO_(3/2).xAIN}.yHfO₂ (0<x<, y is equal to or smaller than0.2).